Is the RTX 5090 overkill for 1440p gaming?

For 1440p at high refresh rates it is more than most games need. It makes sense if you play at 4K, use ray tracing or plan to keep the card for years.

Can it handle video editing and 3D rendering?

Yes. The Ryzen 9, 64 GB of DDR5 and the RTX 5090 cover 4K editing, 3D rendering and large multitasking workloads well.

Do I need to add more RAM or storage?

Not for most buyers. 64 GB of DDR5 and a 4 TB SSD are generous, and both can be expanded later if your work grows.

Yes. The RTX 5090 is well above what current VR headsets require, so it should handle VR games and simulators comfortably.

Is a prebuilt like this worth it over building my own?

You pay a premium over buying the parts separately, but you get a tested, warrantied system with no assembly or troubleshooting. That matters most if you do not want to build.

What should I check about cooling and the power supply?

A flagship GPU and CPU draw a lot of power and heat. Confirm the power supply wattage and the case airflow before buying, since those decide how loud and how fast it runs under load.